The overall purpose of this project is to clarify our understandng of myofibrillar assembly and turnover in the heart. Experiments will utilize both electron microscopic autoradiography and our recent findings which demonstrated the preferential release of newly synthesized myofilaments from myofibrils. Preliminary studies suggest a rapid internalization of myofilament protein following attachment at the fibrillar periphery. We propose to further define the easily released filament population using isolated rat atrial tissue. This in vitro system will permit us to pulse label muscle protein and to test, in a controlled manner, the effects of energy, protein synthesis, insulin branched chain amino acids, microtubules and other factors on myofibrillar assembly and degradation. In addition, patterns of myofibrillar assembly and turnover will be examined during several kinds of cardiac hypertrophy and regression. Finally, we will attempt to demonstrate aspects of myofibrillar assembly and degradation using cell-free preparations.